Pass the t_translation::t_terrain struct by const reference instead of value.
These functions actually made use of the non-const copy that was being made before,
so they needed a few adjustments after making their parameter a const reference.


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56308 75b3775d-3801-0410-9e12-e2e43121521d